This invention relates to a low voltage, multi-pole electric circuit breaker built up by the modular assembly of a breaker unit in a molded insulating box divided into a number of adjacent pole units, and a magneto-thermal tripping unit that is interchangeable according to the selected rating. Each pole of the breaker unit includes a pair of separable contacts, namely a stationary contact linked to an upstream connection terminal of the circuit breaker and a moving contact linked electrically to a connecting strip able to be superimposed in contact with a connecting strip of the tripping unit, the box of which is mounted to rest on the box of the breaker unit. The tripping unit includes thermal and electromagnetic tripping components and is governed by the current flowing in a conductor in line with the pair of contacts in the lengthwise direction of the pole unit. The tripping unit connecting strip is arranged at one of the ends of the conductor while the other end of the conductor is fixed to the downstream connection terminal of the circuit breaker. The electrical continuity at each pole between the conductor of the tripping unit and the pair of contacts of the breaker unit is obtained by means of a connecting screw ensuring the tight contact of the two intermediate tripping and breaker connecting strips when they are superimposed.
It is traditional practice to use a set of magneto-thermal tripping units with different tripping characteristics that can be selectively associated with the sale standard breaker unit in order to constitute circuit breakers having different ratings. A defective tightening of the connecting screw on mounting the tripping unit may give rise to overheating that is detrimental to the proper working of the circuit breaker.
According to a known device of the kind described, the connecting strip of each pole of the breaker unit bears on a stationary contact stud fixed to the base, which contact stud comprises a tapped opening to cooperate with the connecting screw when the pair of tripping unit and breaker unit connecting strips are connected in the superimposed position. It is found that faults in the positioning of the breaker unit and the tripping unit on top of each other, generally due to molding flaws in the plastic, can lead to insufficient tightening of a connecting screw at one of the poles, since all the tapped openings of the stationary contact studs are arranged in the same plane.
To allow for such flaws, a previous improvement has proposed the use of deformable flexible connecting strips of the tripping and/or breaker unit such that their flexibility ensures the appropriate tight contact of the strips when the tripping unit is mounted. However, strips of this type, deformable in design by superimposing conducting foils, are expensive to produce and inadequate for high rating circuit breakers.
The object of this invention is to eliminate these disadvantages and to provide a circuit breaker with an interchangeable tripping unit having a reliable electrical connection whatever the variations in level between the boxes of the breaker unit and the tripping unit.
A feature of a circuit breaker according to the invention is that a flexible connection of the arm of the moving contact is fixed directly to the connecting strip of each pole, which comprises one contact surface cooperating with the corresponding tripping unit connecting strip, while the opposite surface is fitted with a nut, and each breaker unit connecting strip is positioned with vertical play in a transverse slot in the box of the breaker unit; when the connecting screw inserted in the nut is tightened with the tripping unit in the mounted position on the box, it bears on the said floating strip in the direction of the aforementioned play so as to ensure a good electrical contact with the corresponding tripping unit connecting strip, irrespective of any defects in the positioning of the boxes.
According to a development of the invention, the interchangeable tripping unit comprises for each pole an electromagnetic tripping device governed by the current flowing in a conductor in line with the pair of contacts, the tripping unit including a U-shaped magnetic curcuit through which the conductor runs, that cooperates with a moving ferromagnetic vane. This vane is drawn back by a return draw spring anchored at the other end to a bar adjusting the air gap. The air gap is varied by means of a spacer rod linked between the adjusting bar and the vane and lying parallel to the return spring at a small distance away.
Due to the spacer rod, the air gap can be adjusted at the works and the forces exerted by the return spring on the adjusting bar are cancelled out. Due to the relative position of the spacer rod and the spring, any change in the air gap of the electromagnetic tripping unit will result in a constant pulling force of the return spring.